This invention relates to electrical couplings and in particular to moveable contact plunger assemblies which are useful in the electrical coupling of railway or transit cars.
The interconnection of electrical wires from one rail car to the next is achieved by means of connectors arranged in dielectric blocks which are forced together as by clamping. In one transit car design, electrical contact from one wire to another is achieved by means of a butt fit between a moveable contact plunger in one block and a stationary contact pin in the other block at the connection interface.
The system requirements are that the moveable contact plunger be (1) moveable axially so as to forcefully provide the butt fit and (2) be rotatable during axial motion so as to wipe contaminants from the mating surfaces. A further requirement of the system is that the follower of the moveable contact plunger assembly which projects from the other side of the dielectric connector block also be moveable for axial travel so as to provide a force fit with a further connector to the rail car side of the connection interface.
One prior art plunger assembly complying with these requirements consisted of a cylindrical contact plunger with a shoulder that fits in abuttment with a mating shoulder in the aperture of the dielectric block. The plunger terminates at one end in a screw thread area arranged to loosely mate with screw threads carried in a follower element. A spring is arranged concentric with the screw thread area and is seated between a flange at one end of such area and the follower element. The threaded relationship and the spring allows the follower and pin to satisfy the axial travel as well as the rotatable plunger requirements.
Electrical continuity achieved via the loose threading arrangement is rather poor because of a reliance on compressive forces to achieve electrical contact. To compensate the spring is made of a copper alloy so as to provide an additional low resistance current path. These prior art plunger assemblies, though workable, experienced a high failure rate due to over heating of the springs which resulted in tempering and deformation of the spring length and spring rate. This was caused by currents in excess of 75 amperes under worse case as, for example, when a car is connected to a car that has a dead battery so as to present negligible inpedance (short circuit).
Heretofore, attempts to provide moveable contact plunger assemblies with improved failure rate have been unsatisfactory in that they do not comply with the aforementioned contact plunger assembly requirements. One such design employed a solid metallic cylinder extending from the contact pin toward the follower element. This design had the disadvantages of (1) lack of plunger or contact pin rotation, (2) reliance upon compressive force for electrical contact of the cylinder with the follower element and (3) susceptibility to contaminants entering the contact area.
Another design employed a braided wire that was crimped in the plunger contact pin and further attached to the plunger body by screw threads. The same screw threads also served for attachment of the follower element. This design, however, does not provide for plunger rotation and further does not provide for axial deflection of the follower.